An Asymmetric Galactic Stellar Disk Traced by OB-type Stars from LAMOST DR7

Using 9943 OB-type stars from LAMOST DR7 in the solar neighborhood, we fit the vertical stellar density profile with the model including a single exponential distribution at different positions (R, Phi). The distributions of the scale heights and scale length show that the young disk traced by the OB-type stars is not axisymmetric. The scale length decreases versus the azimuthal angle Phi, i.e., from 1.14 (- 0.19) (+ 0.30) kpc with Phi = - 3 degrees to 0.77 (- 0.06) (+ 0.08) kpc with Phi = 9 degrees. Meanwhile we find signal of non-symmetry in the distribution of the scale height of the north and south of the disk plane. The scale height in the north side shows signal of flaring of the disk, while that of the south disk stays almost constant around h( s )= 130 pc. The distribution of the displacement of the disk plane Z( 0) also shows variance versus the azimuthal angle Phi, which displays significant differences with the warp model constrained by the Cepheid stars. We also test different values for the position of the Sun, and the distance between the Sun and the Galactic center affects the scale heights and the displacement of the disk significantly, but that does not change our conclusion that the disk is not axisymmetric.